1. Field of the Invention
This invention relates to a method and apparatus for drilling a secondary borehole from an existing borehole in geologic formations and more particularly, to a tapered window mill and whipstock combination that in one trip, can drill a deviated borehole from an existing earth borehole or complete a side tracking window in a cased borehole.
2. Background
Traditionally, whipstocks have been used to drill a deviated borehole from an existing earth borehole. The whipstock has a ramp surface which is set in a predetermined position to guide the drill bit on the drill string in a deviated manner to drill into the side of the earth borehole. In operation, the whipstock is set on the bottom of the existing earth borehole, the set position of the whipstock is surveyed, the whipstock is properly oriented for directing the drill string in the proper direction, and the drilling string is lowered into the well into engagement with the whipstock causing the whipstock to orient the drill string to drill a deviated borehole into the wall of the existing earth borehole.
Previously drilled and cased wellbores, for one reason or another, may become non-productive. When a wellbore becomes unusable, a new borehole may be drilled in the vicinity of the existing cased borehole or alternatively, a new borehole may be sidetracked from or near the bottom of a serviceable portion of the cased borehole. Sidetracking from a cased borehole is also useful for developing multiple production zones.
Sidetracking is often preferred because drilling, casing and cementing the borehole is avoided. This drilling procedure is generally accomplished by either milling out an entire section of casing followed by drilling through the side of the now exposed borehole, or by milling through the side of the casing with a mill that is guided by a wedge or xe2x80x9cwhipstockxe2x80x9d component.
Drilling a side tracked hole through casing made of steel is difficult and often results in unsuccessful penetration of the casing and destruction of the whipstock. In addition, if the window is improperly cut, a severely deviated dog leg may result rendering the sidetracking operation unusable.
Several patents relate to methods and apparatus to sidetrack through a cased borehole. U.S. Pat. No. 4,266,621 describes diamond milling cutter for elongating a laterally directed opening window in a well casing that is set in a borehole in an earthen formation. The mill has one or more eccentric lobes that engage the angled surface of a whipstock and cause the mill to revolve on a gyrating or non-fixed axis and effect oscillation of the cutter center laterally of the edge thus enhancing the pipe cutting action.
The foregoing system normally requires at least three trips into the well in the sidetracking operation. A first stage begins a window in the casing, a second stage extends the window through use of a diamond milling cutter and a third stage with multiple mills elongates and extends the window. While the window mill is aggressive in opening a window in the casing, the number of trips, such as three, to accomplish the task is expensive and time consuming.
Typically window mills are designed with a square bottom, i.e. a square cross-section. As is shown in FIG. 14, a prior art square bottomed, cross-sectioned mill provides a point of contact between the mill and the whipstock and a large axial surface contact between the mill and the casing. As can be appreciated from FIG. 14, the contact area between the square bottomed mill and whipstock is substantially a line contact while the contact area between the mill and casing is much greater. The applied force, due to the weight on bit, per contact area determines the contact stress between the members. Because the contact stress between the mill and the casing is much greater than the contact stress between the mill and whipstock, the mill tends to cut into the whipstock rather than into the casing even where the cutability of the whipstock has been reduced because of hardfacing.
U.S. Pat. Nos. 2,216,963; 3,908,759; and 4,397,355 disclose mills having a taper or tapered nose. A starter mill with a tapered nose will eventually wedge and cannot complete the window or drill the lateral borehole. U.S. Pat. No. 3,908,759 appears to disclose a taper on the mill. U.S. Pat. No. 2,216,963 discloses a tapered mill which is used in a second trip into the well to increase the window after a square bottomed mill opened the window in a previous trip into the borehole. These patents do not teach guiding and moving these tapered mills laterally through the casing so that at least the center of the downwardly facing cutting surface of these mills passes outside the exterior wall of the casing in one trip into the borehole. At least two trips are required into the well, typically using a starter mil in the first trip to begin cutting a window in the casing and then a second mill in a second trip to increase the window. Further, tapered mills are typically less than full gauge requiring additional into the borehole to complete the window.
Weatherford Enterra offers a mill which has a taper extending upwardly and inwardly from a full diameter cutting base. The mill also includes a support shoulder on the cutting face of the mill. However, the reduced diameter taper extends above the full diameter cutting gage of the mill which therefore tends to cut the whipstock rather than the casing.
U.S. Pat. No. 5,109,924 teaches a one trip window cutting operation to sidetrack a wellbore. A deflection wedge guide is positioned behind the pilot mill cutter and spaced from the end of a whipstock component. The shaft of the mill cutter is retained against the deflection wedge guide such that the milling tool frontal cutting surface does not come into contact with the ramped face of the whipstock. In theory, the deflection wedge guide surface takes over the guidance of the window cutting tool without the angled ramp surface of the whipstock being destroyed.
However, when a second and third milling tool attached to the same shaft as the window milling cutter and spaced, one from the other on the support shaft contacts the whipstock ramped surface, they mill away the deflection guide projection from the ramp surface. This inhibits or interferes with the leading pilot mill window cutter from sidetracking at a proper angle with respect to an axis of the cased borehole and may cause the pilot window cutting mill to contact the ramp surface of the whipstock before the pilot window cutter mill clears the casing. The reamers or mills aligned behind the pilot window mill, having the same or larger diameter than the diameter of the pilot window mill, prevents or at least inhibits the window pilot mill from easily exiting from the steel casing. This difficulty is due to the lack of clearance space and flexibility of the drill pipe assembly making up the one trip window cutting tool when each of the commonly supported reamer mills spaced along the shaft, sequentially contact the window in the steel casing. Hence, the sidetracking apparatus tends to go straight rather than be properly angled through the steel pipe casing.
U.S. Pat. No. 5,445,222 teaches a combination whipstock and staged sidetrack mill. A tapered, cone-shaped mill is located on the end of a common shaft and has an outer diameter of about 50 to 75 percent of the maximum diameter to which the final sidetracked hole will be completed. Three stages of cutting mills are disposed above the tapered mill on the common shaft. Each successive stage increases in diameter. A surface of a second stage cutter is, at its smallest diameter, about the diameter of the maximum diameter of the tapered mill, and is, at its largest diameter, at least 5 percent greater in diameter than the diameter of the tapered mill. A surface of a final stage cutter mill is, at its largest diameter, about the final diameter dimension, and at the smallest cutting surface diameter, is a diameter of at least about 5 percent smaller than the final diameter dimension. The whipstock guide is made of a material that is harder than the casing but not as hard as the cutting elements of the mill whereby the mill is to cut the casing rather than the whipstock.
The sidetracking mill is designed to accomplish the milling operation in one trip. The mill however, tends to go straight and penetrate the ramped surface of the whipstock. Substantial damage to the whipstock occurs and sidetracking may not occur as a result.
While the intent is to perform a sidetracking operation in one trip, difficulties often arise when attempting to deviate the drill string from its original path to an off line sidetracking path. Progressively larger in diameter reaming stages to enlarge the window in the steel casing inhibits the drill shaft from deviating or flexing sufficiently to direct the drill pipe in a proper direction resulting in damage to the whipstock and misdirected sidetracked boreholes. In other words, the sidetracking assembly tends to go straight rather than deviating through the steel casing.
The present invention overcomes these deficiencies in the prior art.
The side tracking system of the present invention includes a window mill having a tapered cutting surface which allows the mill to initiate the cutting of a window into the casing and to move the center of the downwardly facing cutting surface of the mill laterally through the window and past the exterior wall of the casing in one trip into the well without substantially cutting up the whipstock. The tapered cutting surface of the window mill includes taper from a full diameter cutting surface to a reduced diameter cutting surface adjacent the downwardly facing bottom cutting surface of the mill. The mill preferably is used in combination with a whipstock having a ramp which engages the tapered cutting surface of the mill forming a large contact area between the mill and whipstock. The materials of the casing have a first cutablity and the materials of the whipstock have a second cutability.
The tapered cutting surface contacts the whipstock ramp at a first contact area and the full diameter cutting surface of the mill contacts the wall of the casing at a second contact area. As weight is applied to the mill, there is a first contact stress at the first contact area and a second contact stress at the second contact area. The ratio of cutability of the mill with the whipstock and casing is the first cutability divided by the second cutability and the ratio of the contact stress of the mill with the whipstock and casing is the first contact stress divided by the second contact stress. The mill of the present invention cuts the casing rather than the whipstock by maintaining the product of the cutability ratio and the contact stress ratio less than one. This also causes the height of the tapered cutting surface to be at least 50% of the total height, the total height being the distance from the top of the largest diameter cutting surface on the mill to the bottom of the mill.
An object of the present invention is to achieve a cutability ratio times the contact stress ratio of the mill with the whipstock and casing which is less than one such that the mill tends to cut the casing rather than the whipstock. Thus it is a further objective to maximize the contact area between the mill and the whipstock such as by having a tapered cutting surface on the mill and a ramp on the whipstock which has angle substantially the same as the taper of the tapered cutting surface on the mill. Additionally, the contact area is maximized by causing the height of the tapered cutting surface to be at least 50% of the total height of the mill which is the height of the tapered cutting surface and the full diameter cutting surface.
It is an object of this invention to provide a side tracking system which will deflect and move the tapered mill laterally through the casing so that at least the center of the downwardly facing cutting surface of the mill passes outside the exterior wall of the casing in one trip into the borehole. Further it is an object to provide a side tracking system in two trips or less and preferably a one trip cutting system for cutting a deviated hole in an existing earth borehole.
It is another object of this invention to provide a one trip window cutting system for cutting an opening in a pipe casing for subsequent side tracking drilling operations.
More specifically, it is an object of this invention to provide a mill with a tapered cutting end which matches the ramp angle of the whipstock face such that in operation, as the drill string is rotated downwardly, the face of the whipstock forces the tapered cutting end of the window mill out through the pipe casing. The angled face of the whipstock adjacent to the window cutting mill and the cutter mill itself is hardfaced to minimize damage to both the whipstock and the cuter mill.
A one trip side track window cutting apparatus for cutting sidetracking windows in a casing positioned in previously drilled boreholes consist of a window cutting mill affixed to an end of a shaft, a body of the mill forming a tapered cutting end.
A whipstock forms a ramp, the angle of which substantially parallels an angle of the tapered cutting end of the window mill. The ramp acts as a bearing surface for laterally forcing the window mill into the pipe casing. The face of the whipstock changes the rate of deflection of the window mill into the pipe casing.
The whipstock upstream end is ramped about 15xc2x0 to match a 15xc2x0 taper at the end of the window mill cutter. The whipstock upper end is attached to the end of the window mill cutter at the 15xc2x0 interface through a shear bolt extending from a blade of the window mill for installation of the whipstock in a cased borehole. The end of the whipstock is heavily hardfaced, especially adjacent the interface with the window cutter mill. Another mill is positioned upstream of the window mill on the same supporting shaft and is preferably the same diameter as the window mill. When the shear bolt is sheared through an upward force on the drilling string after the whipstock is anchored and properly oriented in the cased borehole, the hardfaced ramp formed by the end of the whipstock forces the window mill immediately into the wall of the casing. Simultaneously, the second mill spaced from the window mill is forced into the casing thus starting two openings in the casing. The whipstock face below the 15xc2x0 ramp parallel the walls of the casing for a distance to allow both the window mill and the second mill to cut the window started by the initial 15xc2x0 ramp. As the window cutting process proceeds, the ramp surface of the whipstock transitions into a xe2x80x9cnormalxe2x80x9d 3xc2x0 ramp for a sufficient distance for the window mill to extend about half way out of the casing where the ramped surface of the whipstock transitions again to a more aggressive angle to further urge the window mill out of the casing.
Once the window mill is centered on the wall of the casing, further cutting becomes difficult because of the reduced rotation of the cutting edges at the center of the tapered window mill. At the exact center of the tapered window mill, there is essentially zero rotation. Thus, in the prior art, it took a long cutting time to have the window mill move and cut past its center line. On a standard 3xc2x0 whip face, it often took a drilling length of plus or minus ten inches to have the center line of the window mill cross the wall of the casing. Very slow drilling progress is made during this period of time because the window mill is attempting to cut the wall of the casing with essentially zero rotation at the center of the window mill.
It is advantageous for all of the mills to be full gage. One advantage is that with your window mill being full gage, the window hole will also be full gage when drilling is stopped with the assembly. If the window mill is undergauged, then when the drilling bit is run into the well, the full gage drilling bit is going to slow down as it cuts the under gage borehole to full gage. This then slows down the operator""s ability to kick off and drill the new borehole with the drilling bit. The drilling bit must remount the bottom section of the borehole cut by the window mill. If the hole is full gage, they will be able to use the whip to help build an angle faster and apply weight to the drilling bit to drill laterally the new borehole. If they have to go down and remount the hole, then they are much further down in the hole before they can kick out for their lateral drilling.
The window mill tapers conform to most of the ramp angles formed by the whipstock. For example, the largest diameter of the window mill forms a 3xc2x0 cutting section matching the 3xc2x0 section of the whipstock below the cylindrical portion of the whipstock. Of course, the 15xc2x0 angle of the window mill is parallel to the 15xc2x0 formed at the top of the whipstock. These matching angulations minimize damage to the whipstock face during the window cutting process thereby assuring a successfully cut window in the casing of the borehole.
After both the window mill and the second mill cut completely through the casing, the window mill is tripped out of the borehole. The sidetracking drilling operation then commences.
An advantage then of the present invention over the prior art is the use of a tapered window mill with a surface contour matching the ramp angle formed at the upstream end of the whipstock such that the mill is forced into the casing immediately after the window mill is released from the whipstock without damage to the whipstock.
Another advantage of the present invention over the prior art is the formation of angled and parallel ramp surfaces formed on the whipstock to facilitate and enhance the cutting action of both the window mill and the second mill, upstream of and spaced from the window mill.
Still another advantage of the present invention over the prior art is the use of an acutely angled ramp section at a point along the ramped whipstock surface when the center of the window mill reaches the inside diameter of the wall of the casing resulting in a slowdown in the window cutting operation. The xe2x80x9ckick outxe2x80x9d ramp more quickly moves the tapered window mill past this phase of the window cutting process thus speeding up the completion of the sidetrack window.
Other objects and advantages of the present invention will appear from the following description.